Electronic device

ABSTRACT

Each of junctions formed between a semiconductor device and a substrate comprises metal balls of Cu, or other materials and compounds of Sn and the metal balls, and the metal balls are bonded together by the compounds.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a continuation application of U.S.application Ser. No. 09/880,773, filed Jun. 12, 2001, which claimspriority from Japanese Patent Applications No. 2000-396905, filed Dec.25, 2000, and No. 2000-180719, filed Jun. 12, 2000.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH OR DEVELOPMENT

[0002] Not applicable

REFERENCE TO A “SEQUENCE LISTING,” A TABLE, OR A COMPUTER PROGRAMLISTING APPENDIX SUBMITTED ON A COMPACT DISK

[0003] Not applicable

BACKGROUND OF THE INVENTION

[0004] The present invention relates to a technique for performingsolder bonding through the use of a temperature hierarchy effective inthe module mounting of electronic devices, and related technology.

[0005] In Sn—Pb-base solders, soldering has so far been performed bytemperature-hierarchical bonding in which parts are soldered first attemperatures between 330° C. and 350° C. by use of high-temperaturesolders, such as Pb-rich Pb-5 mass % Sn (hereinafter the indication of“mass %” is omitted and only a numeral is recited) solders (meltingpoint: 314-310° C.) and Pb-10Sn solders (melting point: 302-275° C.),and bonding is then performed with the aid of Sn-37Pb eutectics (183°C.) of a low-temperature solder without melting soldered portions. Thistemperature-hierarchical bonding is adopted in semiconductor devices inwhich chips are die bonded, and in semiconductor devices of flip chipbonding, and in other types of fabrication processes. In other words, insemiconductor fabrication processes, it has become important to providetemperature-hierarchical bonding between a solder used within asemiconductor device and another solder for bonding the semiconductordevice itself to a substrate.

[0006] On the other hand, in some products there have been cases inwhich bonding at a temperature of not more than 290° C. is requited inconsideration of the heat resistance limit of parts. As solders in acomposition range for high-temperature soldering suited to thisrequirement in conventional Sn—Pb-base solders, a Pb-15Sn solder(liquidus temperature: 285° C.) and solders with similar compositionscan be conceived. However, when the Sn content becomes higher than thislevel, low-temperature eutectics (183° C.) precipitate. Furthermore,when the Sn content becomes lower than this level, the liquidustemperature rises, with the result that bonding at a temperature of notmore than 290° C. becomes difficult. For this reason, even in a casewhere a secondary reflow solder for bonding to a printed circuit boardis a eutectic Sn—Pb-base solder, it has become impossible to avoid theproblem of remelting of bonds of high-temperature solder. When Pb-freesolders are used for secondary reflow, bonding is performed attemperatures of 240-250° C., which are about 20-30° C. higher than witheutectic Sn—Pb-base solders, and therefore the bonding at a temperatureof not more than 290° C. becomes more difficult.

[0007] More specifically, at present there is no high-temperaturePb-free soldering materials that permit temperature-hierarchical bondingat a soldering temperature ranging from 330 to 350° C. or at atemperature level of 290° C.

[0008] This situation is described in detail below. At present, Pb-freesolders are being used in increasingly many applications in terms ofenvironmental issues. In Pb-free solders for soldering to printedcircuit boards, eutectic Sn—Ag-base solders, eutectic Sn—Ag—Cu-basesolders and eutectic Sn—Cu-base solders are going mainstream. As aresult, the soldering temperature in surface mounting is usually in therange of 240 to 250° C. There is no Pb-free solder for a temperaturehierarchy on the higher-temperature side that can be used in combinationwith these eutectic Pb-free solders. As solders that provide the mostsuitable combinations, there are available Sn-5Sb solders (240-232° C.).However, when temperature and other variations on a substrate in areflow furnace, are considered, there is no highly reliable solder onthe lower-temperature side that can perform bonding without melting theSn-5Sb solders. On the other hand, although an Au-20Sn solder (meltingpoint: 280° C.) is known as a high-temperature solder, its use islimited because it is a hard material and its cost is high. Especially,in bonding an Si chip to a material having a substantially differentcoefficient of expansion or in bonding a large-size Si chip, this solderis not used because it is hard and might break Si chips.

BRIEF SUMMARY OF THE INVENTION

[0009] Against the above background, it is required that the necessityof use of Pb-free solders be met and that in mounting a module, afterbonding by use of a higher-temperature side solder at a temperature ofnot more than 290° C., which does not exceed the heat resistance ofparts (primary reflow), the terminal of the module be surface mounted(secondary reflow) to the external connection terminal of a printedcircuit board, by use of Sn-3Ag-0.5Cu solders (melting point: 217-221°C.). For example, a module for a portable product in which chip partsand semiconductor chips are mounted (example: a high-frequency module)has been developed. In this module, the chip parts and semiconductorchips are bonded to the module substrate by use of high-temperaturesolders and cap encapsulation or resin encapsulation is required. Thesechip parts require bonding at a temperature of not more than 290° C.maximum in terms of heat resistance. When the secondary reflow of thismodule is performed by use of an Sn-3Ag-0.5Cu solder, the solderingtemperature reaches about 240° C. Therefore, because even an Sn-5Sbsolder, which has the highest melting point of all Sn-base solders, hasa melting point of 232° C. and because the melting point decreasesfurther when the plating of a chip electrode contains Pb, it isimpossible to avoid the remelting of the soldered portions of the chipparts in the module. Accordingly, a system or process that does not posea problem even when a solder remelts is required.

[0010] It has been conventional practice to perform die bonding of chipsto the module substrate at 290° C. maximum by use of Pb-base solders andto perform the reflow of the chip parts. A soft silicon gel has beenapplied to wire-bonded chips, the top surface of the module substratehas been protected with a cap made of Al, or other materials, andsecondary reflow has been performed by use of eutectic Sn—Pb solders.For this reason, in secondary reflow, stresses are not applied even whenpart of the solder of module junctions melts and, therefore, the chipsdo not move and there is no problem in high-frequency characteristics.However, it becomes necessary to perform secondary reflow by use ofPb-base solders and, at the same time, it has become necessary todevelop a resin encapsulation type module to reduce cost. Therefore, itis necessary to solve the following problems.

[0011] 1) Reflow soldering in air at a temperature of not more than 290°C. must be possible (guaranteed heat resisting temperature of chipparts: 290° C.).

[0012] 2) Melting must not occur in secondary reflow (260° C. maximum)or even if melting occurs, chips must not move (because high-frequencycharacteristics are affected if chips move).

[0013] 3) Even when the solder within the module remelts duringsecondary reflow, a short circuit due to the volume expansion of thesolder of the chip parts must not occur.

[0014] Specifically, problems in the results of an evaluation of an RF(radio frequency) module are stated below. In an RF module, chip partsand a module substrate were bonded together by use of a conventionalPb-base solder (although this solder has a solidus line of 245° C., anSn—Pb-base solder plating is applied to the connection terminals of thechip parts; for this reason, low-temperature Sn—Pb-base eutectics areformed and, therefore, remelting occurs) and the occurrence rate ofshort circuits due to the outflow of the solder after secondary mountingreflow was investigated in the module which was encapusulated so that itwas covered by one operation with various types of insulating resinshaving varied moduli of elasticity.

[0015]FIG. 12(a) is an explanatory drawing of outflow, which show theprinciple of solder flow during the secondary mounting reflow of chipparts in a module. FIG. 12(b) is a perspective view of an example ofsolder flow of chip parts.

[0016] The mechanism of a short circuit due to a solder outflow is suchthat the melting and expanding pressure of a solder within a modulecauses the interfaces between chip parts and a resin or the interfacebetween the resin and a module substrate to be exfoliated and the solderflows into the exfoliated interface(s), with the result that theterminals at both ends of a surface mounted part are connected to eachother, causing a short circuit.

[0017] As a result of the above investigation, it became apparent thatthe incidence of short circuits due to a solder outflow is in proportionto the modulus of elasticity of resins. It became also apparent thatconventional high-elasticity epoxy resins are inappropriate and that inthe case of soft silicone resins, short circuits do not occur when themodulus of elasticity at 180° C. (melting point of Sn—Pb eutectics) islow.

[0018] However, because low-elasticity resins in practical use aresilicone resins, during the process of substrate dividing, part of theresins cannot be completely divided and, in some cases, remain forreasons of resin properties. Therefore, a process of making cuts by alaser, or other apparatus, becomes necessary. On the other hand, in thecase of general epoxy resins, mechanical dividing is possible althoughshort circuits occur because of hardness and these resins areinappropriate. However, in terms of resin properties, it is at presentdifficult to make the resins soft to such an extent that short circuitsdoe not occur at 180° C. If it is possible to perform resinencapsulation that serves as mechanical protection and can, at the sametime, prevent a solder outflow, covering with a case or cap isunnecessary and, therefore, cost can be reduced.

[0019] The present invention provides a completely new solder paste, amethod of solder bonding, and a soldered joint structure. It alsoprovides temperature-hierarchical bonding by use of a solder capable ofmaintaining bonding strength at high temperatures. In addition theinvention provides an electronic device in whichtemperature-hierarchical bonding is performed by use of a solder capableof maintaining bonding strength at high temperatures.

[0020] Representative features of the invention disclosed in thisapplication to achieve the above objects are summarized below. In theinvention, as a solder for bonding electronic parts and a substratetogether, a solder paste containing Cu balls and Sn solder balls isused.

[0021] According to the invention, there is provided an electronicdevice provided with electronic parts and a substrate, in which the padsof the electronic parts and the pads of the substrate are bonded byjunctions each containing Cu balls and a compound of Cu and Sn, and theCu balls are bonded together by the compound of Cu and Sn.

[0022] Further, according to the invention, in an electronic device inwhich a primary substrate having electronic parts mounted thereon ismounted on a secondary substrate such as a printed circuit board andmother board, the bonding of the electronic parts to the primarysubstrate is performed by the reflow of the solder paste containing theCu balls and Sn solder balls and the bonding of the primary substrate tothe secondary substrate is performed by the reflow of anSn-(2.0-3.5)Ag-(0.5-1.0)Cu solder.

[0023] For example, regarding temperature-hierarchical bonding, evenwhen a part of solder on the higher-temperature side that has alreadybeen bonded melts, the solder can provide strength high enough towithstand a process during later solder bonding if other portion doesnot melt.

[0024] The melting points of intermetallic compounds are high. Becauseportions bonded with intermetallic compounds can provide sufficientbonding strength even at 300° C., intermetallic compounds can be usedfor temperature-hierarchical bonding on the higher-temperature side.Therefore, the present inventors performed bonding by use of a pastewhich is a mixture of Cu (or Ag, Au, Al or plastic) balls or these ballswhose surfaces are plated with Sn, or other materials, and Sn-basesolder balls, both of them being mixed at a volume ratio of about 50% toabout 50%. As the result, in portions where the Cu balls are in contactwith each other or in close vicinity to each other, a reaction withsurrounding molten Sn occurs and a Cu₆Sn₅ intermetallic compound isformed because of diffusion between Cu and Sn, making it possible toensure sufficient bonding strength between the Cu balls at hightemperatures. Because the melting point of this compound is high andsufficient strength is ensured at a soldering temperature of 250° C.(only the Sn portion melts), no exfoliation of bonded portions occursduring secondary reflow performed for mounting onto a printed circuitboard. Therefore, the soldered portions of a module are made of acomposite material having two functions, that is, the first function ofensuring high-temperature strength during the secondary reflow byelastic bonding force brought about from the bonding of the high-meltingpoint compound and the second function of ensuring service life by theflexibility of soft Sn during temperature cycles. Therefore, thesoldered portions can be adequately used in the temperature-hierarchicalbonding at the high temperatures.

[0025] Furthermore, also in the case of hard and high-rigidity soldershaving desirable melting points, such as an Au-20Sn solder, Au-(50-55)Snsolders (melting point: 309-370° C.) and an Au-12Ge (melting point: 356°C.), by using granular particles and dispersing and mixing soft andelastic rubber particles or by dispersing and mixing soft low-meltingpoint solders of Sn, In, etc., among the above hard and high-rigiditysolders, it is possible to ensure bonding strength even at temperaturesof not less than the solidus temperatures of the above hard andhigh-rigidity solders and to relieve phenomena caused due todeformation, by the soft Sn, In or rubber among metal particles, wherebysuch a new effect as to compensate for the drawbacks of solders can beexpected.

[0026] Next, a resin-encapsulated RF module structure is discussed.Conceivable measures to prevent shorts from being caused by a solderinclude (1) adopting a structure in which the solder within the moduledoes not melt in secondary mounting reflow and (2) adopting anotherstructure in which even when the solder within the module melts, theexfoliation at the interfaces between parts and the resin and at theinterface between the resin and the module substrate is prevented byreducing the melting-and-expanding pressure of the solder. However,resin design is difficult in these measures.

[0027] On the other hand, (3) relieving the melting-and-expandingpressure of a molten internal solder by use of a low-hardness resin in agel state, is conceivable. However, because of a small protective force(mechanical strength), covering with a case or cap is required. Thismeasure cannot be adopted because of cost.

[0028]FIG. 13 (described later) shows a comparison of phenomena ofmolten solder flow between a case where a conventional solder is used ina resin encapsulation structure and another case where the solder of theinvention is used. The volume expansion of Pb-base solders is 3.6%[Science and Engineering of Metallic Materials; Masuo Kawamori, p.14442]. In a bonding structure of the invention, only Sn melts attemperatures of about 240° C. during secondary reflow mounting.Therefore, in view of the fact that the volume ratio of Cu balls to Snballs is about 50%, the volume expansion immediately after melting is1.4%, which is about 1/2.5 of that of Pb-base solders. On the otherhand, regarding the state of remelting, the conventional solderinstantaneously expands 3.6% when it remelts. Therefore, in the case ofa hard resin, since the resin cannot be deformed, the pressureincreases, with the result that the molten solder flows into theinterfaces between the chip parts and the resin. For this reason, it isnecessary that the resin be soft. On the other hand, with the solder ofthe invention, as is apparent from a model of the section of a chipshown in FIG. 1, Cu particles are bonded together mainly via Cu₆Sn₅compounds. Even when the Sn in the gap among Cu particles melts, the Cuparticles do not move because they are bonded together. Therefore, thepressure by the resin can be coped with by the reaction force of thebonded Cu particles, with the result that a pressure is not easilyapplied to the molten Sn. Further, since the volume expansion of thebonded portion is as low as 1/2.5 of that of the conventional solder, itis expected that, because of the synergistic effect of both of them, thepossibility that Sn flows over the interfaces of chip parts is low.Thus, by adopting the bonding structure of the invention in this module,it is possible to provide a low-cost RF module which can be encapsulatedwith a somewhat softened epoxy resin and which, at the same time, can beeasily cut.

BRIEF DESCRIPTION OF THE DRAWINGS

[0029]FIG. 1 is a sectional view of a model showing the material andconstitution of a paste for bonding.

[0030]FIG. 2(a) shows a model of a section regarding an example to whichthe invention is applied and

[0031]FIG. 2(b) and FIG. 2(c) are model views of a method of pastesupply and a bonded condition, respectively.

[0032]FIG. 3(a) and FIG. 3(b) are sectional views of a case where theinvention is applied to a surface etching pattern.

[0033]FIG. 4 is a sectional view before bonding in a case where theinvention is applied to plating easily capable of alloying.

[0034]FIG. 5(a) to FIG. 5(c) are sectional views of a model in which amodule is mounted on a printed circuit board.

[0035]FIG. 6 is a sectional view of a model of plastic package.

[0036]FIG. 7(a) to 7(c) are sectional views of a model of mounting an RFmodule.

[0037]FIG. 8(a) and FIG. 8(b) are process flow charts of RF modulemounting.

[0038]FIG. 9(a) to FIG. 9(d) are sectional views of a model of processsequence of an RF module.

[0039]FIG. 10 is a perspective view of the mounting state of an RFmodule on a mounting substrate.

[0040]FIG. 11 is a perspective view of a method of resin printing in theassembling of an RF module.

[0041]FIG. 12(a) and FIG. 12(b) are a sectional view and a perspectiveview, respectively, of the principle of solder flow in a comparativeexample of RF module.

[0042]FIG. 13 shows a comparison of the phenomena of RF module between acomparative example and a example relating to the invention.

[0043] FIGS. 14(a) to 14(c) are a plan view of a high-output resinpackage and a sectional view of the package.

[0044]FIG. 15 is a flow chart of the process of a high-output resinpackage.

[0045]FIG. 16(a) to FIG. 16(d) are sectional views of a model of CSPjunctions obtained by the bonding of composite balls.

[0046]FIG. 17(a) to FIG. 17(c) are sectional views of a model of BGA/CSPin which Cu ball bumps are used.

[0047]FIG. 18(a) to FIG. 18(c) are sectional views of a model of BGA/CSPin which Cu-coated bumps of deformed structure are used.

[0048]FIG. 19 shows the relationship between the Sn/Cu ratio and anappropriate range of bonding.

DETAILED DESCRIPTION OF THE INVENTION

[0049] Embodiments of the invention are described below.

[0050] Embodiment 1

[0051]FIG. 1 shows the concept of a bonding structure relating to theinvention. This figure also shows a condition before soldering andanother condition after soldering. The upper portions of FIG. 1 shows anexample of use of a paste in which Cu balls 1 with a particle size ofabout 30 μm (or balls of Ag, Au, Cu—Sn alloys, or other materials, orthose to which an Au plating, and Ni/Au plating, are applied, or thoseto which an Sn plating, or other materials are applied) and Sn solderballs 2 (melting point: 232° C.) with a particle size of about 30 μm areappropriately dispersed in small quantities via a flux 4. When thispaste is subjected to reflow at a temperature of not less than 250° C.,the Sn solder balls 2 melt, molten Sn 3 spreads so that it wets the Cuballs 1 and becomes present between the Cu balls 1 relatively uniformly.The Cu balls need not be spherical; that is, Cu balls having greatsurface irregularities, bar-like ones, and those containing dendritecrystals may be used. In this case, the cubic ratio of Cu to Sn isdifferent and it is only necessary that a Cu ball be in contact with anadjoining Cu ball. The superiority of the spherical shape lies inprintability. After bonding, to ensure strength at high temperatures, itis necessary that the Cu balls be entangled with each other. On theother hand, if the Cu balls are too much constrained by each other tomove, there is no degree of freedom in soldering and deformability isinsufficient, posing a problem. Regarding this respect, it seems idealthat the Cu balls in which dendritic crystals are linked by contact withthe result that elastic motion occurs. Thus, there is a method in whichdendritic crystals of Cu are wrapped with Sn, etc., and are then spheredand the spheres are mixed. Incidentally, the particle size of the Cu andSn balls is not limited to about 30 μm.

[0052] Because Cu₆Sn₅ compounds are formed in a short time by using areflow temperature as high as possible, the aging process for theforming of the compound becomes unnecessary. When the forming of theCu₆Sn₅ compound is insufficient, it is necessary to ensure the strengthof bonding between the Cu balls 1 by performing short aging in atemperature range of the heat resistance of the parts. Because themelting point of the Cu₆Sn₅ compound is as high as about 630° C. and themechanical properties of the Cu₆Sn₅ compound are not poor, there is noproblem in strength. If the aging is performed for a long time at a hightemperature, Cu₃Sn compound comes to grow to the Cu side. It is thoughtthat, regarding mechanical properties, Cu₃Sn is generally deemed to behard and brittle. However, even when Cu₃Sn is formed within the solderaround each of the Cu particles, there is no problem insofar as it hasno effect on service life measured in a temperature cycle test, or othertests. In an experiment in which Cu₃Sn was sufficiently formed at a hightemperature in a short time, there was no problem in strength. It isthought that this is because there is a difference in the effect ofCu₃Sn on fracture between such a case where Cu₃Sn is formed long alongthe bonding interface as to have so far been experienced and suchanother case where Cu₃Sn is formed around each of the particles as inthis example. It is also thought that in the present case, asupplementary effect of soft Sn present around the compound is alsogreat.

[0053] Since, as disclosed above, the Cu balls 1 are bonded to eachother via the compounds (Cu₆Sn₅), neither junctions (Cu₆Sn₅) nor Cuballs 1 melt, it becomes possible to provide the bonding strength evenwhen the module passes through a reflow furnace at about 240° C. afterbonding. In taking the reliability of bonding among the Cu balls 1 intoaccount, it is preferred that the compounds (Cu₆Sn₅) be formed with athickness of about a few micrometers. Further, to ensure that thecompounds are formed among the Cu balls 1, it is preferred the gapbetween the Cu balls be short so that they almost come into contact witheach other. This is made possible by adjusting the amount of Sn.However, it is not necessary that all adjoining Cu particles be bondedtogether by the compound. Instead, in terms of probability, it ispreferred that portions where linkage occurring by the compound does notoccur are present, because this provides a degree of freedom indeformation. When the Cu balls are constrained within a region, there isno problem in strength. Incidentally, the flux 4 may be any one of acleaning type and a non-cleaning type.

[0054] The upper portions of FIG. 1 shows another example in which theabove Cu balls 1 are plated with Sn, or other materials, of a fewmicrometers in thickness. When the amount of Sn is insufficient due tothin Sn plating, the insufficient amount of Sn is compensated for by theSn balls of the same ball diameter. The Sn plating of Cu enables themolten Sn 3 to readily spread along the balls and wet them, making thegaps among the Cu balls 1 more even. Further, this has also a greateffect on the elimination of voids. The oxide film of the solder platingis broken during reflow and the Cu balls are sucked by each other underthe action of surface tension and approach each other. Further, thefluidity of the solder is improved by adding a trace amount (1-2%) of Bito Sn to thereby improve the wettability of the solder onto terminals.Addition of a large amount of Bi is undesirable because the solderbecomes brittle.

[0055] Next, electronic parts such as LSI packages and parts having thisbonding structure are mounted on a printed circuit board. In thismounting, temperature-hierarchical bonding becomes necessary. Forexample, after printing of an Sn-3Ag-0.5Cu solder paste (melting point:221-217° C.) on connection terminals of a printed circuit board andmounting of electronic parts such as LSI packages and parts, reflow canbe performed at 240° C. in the air (this is possible also in a nitrogenatmosphere). This Sn-(2.0-3.5)Ag-(0.5-1.0)Cu solder is treated as astandard solder that replaces conventional eutectic Sn—Pb solders.However, because this solder has a higher melting point than theeutectic Sn—Pb solders, it is required that a high-temperature Pb-freesolder suitable for this purpose be developed. As mentioned above, thestrength at the high temperatures is ensured between Cu and Cu₆Sn₅ inalready formed junctions and the strength of the junctions is highenough to withstand stresses caused by the deformation of a printedcircuit board during reflow, or other processes. Therefore, even when anSn-(2.0-3.5) Ag-(0.5-1.0)Cu solder is used for the secondary reflow forsoldering to a printed circuit board, this solder can realizetemperature-hierarchical bonding because it has such a function as to bea high-temperature solder. In this case, the flux to be used may be anRMA (rosin mild activated) type for non-cleaning application or an RA(rosin activated) type for cleaning application, and any one of both ofthe cleaning type and the non-cleaning type can be used.

[0056] Embodiment 2

[0057] In FIG. 2(a), a semiconductor device 13 is bonded to a junctionsubstrate 6 by use of an Au-20Sn or other solder, and after wire bonding8, the peripheral portion of a cap 9, which is fabricated by plating Al,Fe—Ni, with Ni—Au, is bonded 10 to the junction substrate by reflowthrough the above solder paste of non-cleaning type. On this occasion,when the insulating characteristic is regarded as important, it isdesirable to perform bonding in a nitrogen atmosphere by use of a solderwith a flux not containing chlorine. However, when the wettabilitycannot be ensured, the encapsulation with a weak-activity rosin of RMAtype may be performed. This semiconductor device is not required to havea perfect encapsulatability and, therefore, if the flux has adequateinsulating characteristics, there occurs no influence of holding for along time on the semiconductor device even in the presence of the flux.The purpose of the cap encapsulation is mainly mechanical protection. Asa method of the encapsulation, it is possible to perform the pressurebonding of a sealing portion by means of a pulse-current resistanceheating body 15, In this case, the application of the paste is performedalong the sealing portion by means of a dispenser and a fine continuouspattern 12 are formed (FIG. 2(b)).

[0058] A model in which the section A-A′ of the pattern is enlarged isshown on the right side. Cu balls 1 and Sn balls 2 are held by a flux 4.When bonding is performed by means of the pulse-current resistanceheating body 15 while applying a pressure from above, the paste is madeflat as shown in FIG. 2(c). Section B-B′ which is made flat is enlargedon the right side. In this case, when Cu balls of 30 μm are used, thesolder bonding portion between a junction substrate 6 and a cap 9provides a gap corresponding to 1 to 1.5 Cu ball (about 50 μm). Becausebonding under pressure by means of the pulse heater was performed at350° C. for 5 seconds, the contact portion between the Cu ball 1 and theterminal of the junction substrate 6 and the contact portion between theCu ball and the cap 9 readily form Cu₆Sn₅ or Ni₃Sn₄ compounds in a shorttime insofar as a thick Cu-base or Ni-base plating layer is formed onthe cap surface. In this case, therefore, the aging process is generallyunnecessary. When a narrow paste application width is intentionallyadopted, for example, when the paste is applied under pressure to asection of 250 μm in width×120 μm in height, the thickness of thesection becomes equivalent to the thickness of 1 to 1.5 particle afterpressure application and hence it comes to spread to a width of about750 μm.

[0059] Eutectic Sn-0.75Cu solder balls are supplied beforehand to thisencapsulated package as external junction terminals 11, and on theprinted circuit board, the solder paste is positioned and provided, aswith other parts, by printing, and the surface mounting is performed byreflow. As a reflow solder, any one of an Sn-3Ag solder (melting point:221° C., reflow temperature: 250° C.), an Sn-0.75Cu solder (meltingpoint: 228° C., reflow temperature: 250° C.), Sn-3Ag-0.5Cu solders(melting point: 221-217° C., reflow temperature: 240° C.), and, may beused. Because as is apparent from the results so far obtained,sufficient strength is ensured between Cu and Cu₆Sn₅ by the eutecticSn—Pb solder, encapsulated portions, are not exfoliated during reflow.Incidentally, when a lap-type joint portion obtained by bonding piecesof Cu foil together by use of this solder paste was subjected to ashearing tensile test (tensile rate: 50 mm/min) at 270° C., the value ofabout 0.3 kgf/mm² were obtained. This reveals that a sufficient strengthat high temperatures is ensured in the junction.

[0060] In the case of a module whose cap portion is made of Al platedwith Ni—Au or made of Fe—Ni plated therewith, the growth rate of anNi—Sn alloy layer at a temperature of not less than 175° C. is higherthan that of a Cu—Sn alloy layer insofar as the Ni-containing layer isformed with a film thickness of about 3 μm (for example, D. Olsen etal.; Reliability Physics, 13th Annual Proc., pp 80-86, 1975) and,therefore, an Ni₃Sn₄ alloy layer is also sufficiently formed byhigh-temperature aging. However, as regards the properties of the alloylayer, Cu₆Sn₅ is superior. Thus, it is not preferred to make the Ni₃Sn4alloy layer grow to have a thicker thickness. In this case, because thehigh-temperature aging cannot be made to be a long period of time, thereoccurs no problem of embrittlement caused due to excessive growth. Fromdata on an Sn-40Pb solder which has a lower growth rate of an alloylayer than Sn and which has been used in actual operations, it ispossible to predict an outline of the growth rate of Sn. The growth rateof occurring by Ni and Sn-40Pb is not more than 1 μm even at 280° C. in10 hours (according to some data, the growth rate is 1 μm at 170° C. in8 hours). Thus, no problem of the embrittlement occurs insofar as thehigh temperature aging is short in the period of time. As regards thegrowth rate of the alloy layer (Ni₃Sn₄) occurring by Sn and the Niplating, it is known that the growth rate thereof differs greatly independence on the types of plating such as electroplating and chemicalplating, or other materials. Because it is necessary to keep the highbonding strength, a high growth rate of the alloy layer is desired inthe embodiment. On the other hand, there is such a data as the growthrate of Cu₆Sn₅ occurring by Cu and the Sn-40Pb solder is 1 μm at 170° C.in 6 hours (which corresponds to a growth rate of 1 μm per one hour at230° C. in the case of the Sn-0.75Cu eutectic solder balls used in theembodiment on the assumption that the solder balls are simply in a solidstate). In a bonding experiment at 350° C. for 5 seconds, the inventorswere able to observe portions where Cu₆Sn₅ of 5 μm maximum in thicknesswere formed between Cu particles. From this fact, it is deemed that noaging process is generally necessary when the soldering is performed atthe high temperature.

[0061] In this paste method, it is also one of the most importantproblems to reduce the occurrence of voids as little as possbile. Forthis reducing, it is important to improve the wettability of the solderfor the Cu particles and to improve the fluidity of the solder. Forachieving this, the Sn plating on the Cu balls, Sn—Cu solder platingthereon, Sn—Bi solder plating thereon and Sn—Ag solder plating thereon,the adoption of eutectic Sn-0.7Cu solder balls, Bi addition to solderballs, are each effective means.

[0062] Further, the solder balls are not limited to Sn solder balls, andsolder balls may be used in which any one or more of eutectic Sn—Cu-basesolder balls, eutectic Sn—Ag-base solder balls, eutectic Sn—Ag—Cu-basesolder balls, and solder balls obtained by adding at least one elementselected from In, Zn, and Bi to any one of these solder balls may beused. Also, in these cases, because Sn is the main element of thecomposition, the desired compound can be formed. Also, two or more kindsof balls may be mixed. Because in these balls the melting point of theseballs is lower than Sn, the growth rate of the alloy layer at hightemperatures generally tends to become high.

[0063] Embodiment 3

[0064] The paste relating to the invention can also be used in the diebonding 7 shown in FIG. 2(a). After bonding by use of the paste relatingto the invention, cleaning and wire bonding are performed. In priorarts, the die bonding is performed by use of an Au-20Sn solder, however,it has been limited to small chips in view of reliability. Also, in thecase of Pb-base solders, a Pb-10Sn solder has been used. The bondingrelating to the invention can be used even in chips having a somewhatlarger area. The larger the thickness of a bonding portion, the longerthe service life and the higher the reliability become. In theinvention, it is possible to make this thickness larger by using highmelting point balls each having a larger size. In a case of making thisthickness small, this is performed by making the size of particles (thatis, balls) small. In some bonding methods, it is also possible to use athin bonding portion by reducing the particle size. Even Cu particlesizes of 5-10 μm may be also used, and particles of further smaller sizemay be mixed therewith. The compounds occurring between an Si chip(Cr—Cu—Au, Ni plating, or other materials is provided as the metallizedlayer of the back side thereof) and Cu balls and between Cu balls andthe connection terminal on the substrate include Sn—Cu compounds andSn—Ni compounds. Since the growth rate of the alloy layer is small, noproblem of embrittlement occurs.

[0065] Embodiment 4

[0066] The junction provided by a high-temperature solder needs towithstand only during reflow which is performed in a succeeding step,and the stress applied to this junction during the reflow is thought tobe small. Therefore, instead of using the metal balls, one side or bothsides of each of connection terminals are roughened so that projectionsof Cu, Ni, or other materials may be formed, whereby an alloy layer isformed surely at the contact portions of the projections and otherportions are bonded with a solder. This provides the same effect as withthe balls. The solder is applied to one of the terminals by means of adispenser, the solder being then made to melt while the projections aremade from the above to be forced to intrude into each other by means ofa resistance heating body of pulsed electric current, whereby the diebonding is performed at a high temperature. As a result, because of theanchor effect of the projections and the formation of the compounds inthe contact portions, the contact portions can obtain strength highenough to withstand the stress occurring during the reflow. FIG. 3(a)shows a model of the section of a junction in which the surface of a Cupad 18 of a substrate 19 is roughened by etching and a paste of anSn-base solder 2 is applied to the roughened surface. In this case, fineCu particles may be added to the Sn-base solder. The back side of aterminal portion 75 of a part may be flat. In this case, however, it wasplated with Cu or Ni, and the surface thereof was roughened by etching20. FIG. 3(b) shows a state of the bonding performed by heating underpressure. The compounds are formed in the contact portions by the reflowperformed at a somewhat high temperature, so that the contact portionbecomes strong in strength. Therefore, in the succeeding reflow step inwhich the external connection terminals are bonded to the terminals ofthe substrate, this portion is not exfoliated.

[0067] Embodiment 5

[0068] In bonding by use of Au—Sn alloys in which the amount of diffusedelements is increased by aging and regarding which resultant compoundschange in about three stages from a low-temperature to a high-meltingpoint side, various compounds are formed at relatively low temperatureswithin a range of a small temperature variation. A well-knowncomposition of the Au—Sn alloy is Au-20Sn (melting point: 280° C.,eutectic type). The composition range of Sn in which the eutectictemperature of 280° C. is maintained is from about 10 to 37 mass % Sn.There occurs a tendency to become brittle when the Sn content thereofincreases. It is deemed that a composition range which may be realizedin an alloy containing Au of a low content is 55 to 70% Sn, and in thiscomposition a 252° C.-phase comes to be present (Hansen; Constitution ofBinary Alloys, McGRAW-HILL, 1958). Since the possibility that thetemperature of a portion bonded in the preceding step (primary reflow)reaches 252° C. after the bonding in a succeeding step (secondaryreflow) is thought to be low, it is thought that even in thiscomposition range, the purpose of temperature-hierarchical bonding canbe achieved. As regards the compositions, AuSn₂ and AuSn₄ are formed,which can be applied to the die bonding or to the encapsulation portionof the cap. For further extra safety, an Au—n alloy containing Sn of 50to 55 mass % may be adopted, in which alloy the solidus line and theliquidus line thereof become 309° C. and maximum 370° C., respectively,so that it becomes possible to prevent the 252° C.-phase from occurring.FIG. 4 shows a model of a section in which the back side of an Si chip25 is plated beforehand with Ni(2 μm)-Au (0.1 μm), for example, taps ona lead frame 19 being plated with Ni (2 μm)22-Sn (2-3 μm)23. In the diebonding performed in a nitrogen atmosphere while heating under pressureand in the aging additionally applied as occasion required, a part of Snis consumed to form the Ni—Sn alloy layer (, that is, the Ni—Sn compoundlayer)and the remainder of Sn forms an Au—Sn alloy phase. In a casewhere the Sn content is too high, a low eutectic point (217° C.) of Snand AuSn₄ is formed. Therefore, it is necessary to control the Sncontent so that this eutectic point may be not formed. Alternatively, apaste in which fine metal particles and Sn, are mixed may be coatedthereon. Because the die bonding by use of Au—Sn solders is performed ata high temperature of 350-380° C., it is possible to form a compound inwhich the Sn content is lower than that of the AuSn₂, by controlling thefilm thickness, temperature and a period of time, whereby the meltingpoint thereof can be made to be not less than 252° C. Thus, it isthought that no problem occurs in the succeeding reflow step.

[0069] As mentioned above, by causing the solder to melt at 300° C.levels, which are considerably higher than the melting point of Sn, thediffusion of the elements was activated and the compound were formed,whereby the strength required at the high temperature was able to beensured and the high-reliability bonding thereof on thehigher-temperature side in the temperature-hierarchical bonding was ableto be realized.

[0070] As regards the metal balls described above, there may be used anyone of the balls of single-element metal (for example, Cu, Ag, Au, Aland Ni), the balls of alloy (for example, Cu alloy, Cu—Sn alloy andNi—Sn alloy), the balls of compounds (for example, Cu₆Sn₅ compound) andthe balls that contain mixtures of the above. In other words, there canbe used any kind of substance in which compounds are formed with moltenSn so that the bonding between metal balls can be ensured. Therefore,metal balls are not limited to one type, and two or more types of metalballs may be mixed. These metal balls may be treated by Au plating, orNi/Au plating, or single-element Sn plating, or alloy plating containingSn. Further, resin balls whose surfaces are plated with one kindselected from Ni/Au, Ni/Sn, Ni/Cu/Sn, Cu/Ni and Cu/Ni/Au may be used. Astress relieving action can be expected from mixing the resin balls.

[0071] Embodiment 6

[0072] Next, there is described a case where Al balls are used as ballsof other metals. In general, high-melting metals are hard, and pure Alis available as a metal that is inexpensive and soft. Pure Al (99.99%)usually does not wet Sn although it is soft (Hv 17). However, Sn can bereadily wetted by plating the pure Al with Ni/Sn, Ni/Cu/Sn, or othermaterials. The pure Al readily diffuses at a high temperature in avacuum. Therefore, by using Sn-base solders containing Ag under somebonding conditions, it is possible to form compounds with Al, such asAl—Ag. In this case, the metallization of the Al surface is unnecessaryand this provides a great cost merit. Trace amounts of Ag, Zn, Cu, Ni,may be added to Sn so that Sn reacts readily with Al. The Al surface canbe wetted either completely or in spots. When stress is applied in thelatter case of spots-like wetting, because restraining from deformingbecomes small insofar as a bonding strength is ensured, deformability isgood and the unwetted portions absorb energy as friction loss.Therefore, a material excellent in deformability is obtained. It is alsopossible to plate an Al wire with Sn, Ni—Sn, Ag, and divide it into aparticle form. Al particles can be produced in large amounts at low costby the atomization process, etc., in a nitrogen atmosphere. It isdifficult to produce Al particles without surface oxidation. However,even when the surface is once oxidized, oxide films can be removed bythe metallizing treatment.

[0073] Embodiment 7

[0074] Next, Au balls are described. In the case of the Au balls, Snreadily wets them and, therefore, metallizing is unnecessary insofar asbonding performed in a short time is concerned. However, in a case wherethe soldering time is long, Sn diffuses remarkably and there occurs sucha fear as brittle Au—Sn compounds are caused. For this reason, to ensurea soft structure, In (indium ) plating in which the degree of diffusionto Au is low, etc., is effective, and Ni, Ni—Au, used as a barrier arealso effective. Forming a barrier layer that is made to be as thin aspossible makes Au balls readily deformable. Alternatively, othermetallized structures may also be adopted insofar as they can suppressthe growth of an alloy layer with Au. When the bonding is performed in ashort time in the die bonding, an alloy layer formed at grain boundariesis thin in thickness, so that the effect brought about by theflexibility of Au can be greatly expected even in a case where nobarrier is provided. The combination of the Au balls and In solder ballsis also possible.

[0075] Embodiment 8

[0076] Next, Ag balls are described. The case of the Ag balls is alsosimilar to that of the Cu balls. Since the mechanical properties ofAg₃Sn compounds such as hardness are not poor, so that it is alsopossible to perform the bonding of the Ag particles together through thecompounds by a usual process. It is also possible that Ag balls aremixed in the Cu.

[0077] Embodiment 9

[0078] Next, there is described a case where a metal material are usedas metal balls. As representative alloy-base materials, Zn—Al-base andAu—Sn-base materials are available. The melting point of a Zn—Al-basesolder are mainly in the range from 330° C. to 370° C., which aresuitable for performing the hierarchical bonding with a Sn—Ag—Cu-basesolder, a Sn—Ag-base solder and a Sn—Cu-base solder. As representativeexamples of the Zn—Al-base solder, there are available a Zn—Al—Mg-basesolder, a Zn—Al—Mg—Ga-base solder, a Zn—Al—Ge-base solder, aZn—Al—Mg—Ge-base solder, and any one of these solders that contains atleast one kind selected from Sn, In, Ag, Cu, Au, and Ni. In the case ofthe Zn—Al-base solder, the oxidation thereof occurs intensively and thesolder rigidity thereof is high. For these reasons, it is pointed outthat cracks may occur in Si chips when Si is bonded (Shimizu et al.:“Zn—Al—Mg—Ga Alloys for Pb-Free Solders for Die Attachment,” Mate 99,1992-2). Thus, these problems must be solved when the Zn—Al-base solderis used as metal balls.

[0079] Accordingly, in order to solve these problems, that is, in orderto lower the rigidity of the solders, heat-resistant plastic ballsplated with Ni/solder, or Ni/Cu/solder, or Ni/Ag/solder or Au wereuniformly dispersed in the Zn—Al-base balls to thereby lower Young'smodulus. It is preferred that each of these dispersed particles besmaller in size than that of the Zn—Al-base balls and be uniformlydispersed in the Zn—Al-base balls. At the time of the deformation, thesoft plastic balls with elasticity having a size of about 1 μm deform,so that there is brought about a great effect on the relieving ofthermal impact and mechanical impact. When a rubber is dispersed in theZn—Al-base solder balls, the Young's modulus decreases. Since theplastic balls are almost uniformly located among the Zn—Al-base solderballs, this dispersion is not greatly varied during the meltingperformed in a short time. Further, by using plastic balls whose thermaldecomposition temperature is about 400° C., the organic substancesthereof can be prevented from being decomposed in the solder during thebonding performed through the resistance heating body.

[0080] The Zn—Al is apt to be readily oxidized. Thus, in taking thestorage and handling thereof into consideration, it is preferred thatthe surface thereof be plated with Sn formed by replacing Cu. The Sn andCu dissolve during the bonding in the Zn—Al solder insofar as a lowamount of Sn and Cu is concerned. Because of the presence of Sn on thesurface, it become easy to perform, for example, the bonding onto anNi/Au plating formed on a Cu stem. At such a high temperature as to benot less than 200° C., the growth rate of an Ni—Sn alloy layer (Ni₃Sn₄)is larger than that of Cu₆Sn₅, so that there occurs no phenomenon thatthe bonding were impossible due to insufficient formation of thecompounds.

[0081] Further, by making the Sn balls of 5-50% mixed therein inaddition to the plastic balls, Sn layers come to be located among theZn—Al-base solders. In this case, a part of the Zn—Al balls are bondeddirectly to each other, however, in the other portions, relatively softSn—Zn phase of a low melting point and remaining Sn and other materialscome to be present, so that any deformation can be absorbed by the Sn,the Sn—Zn phase and the rubber of the plastic balls. In particular,because of a combined action of the plastic balls and the Sn layers,further relieving of the rigidity can be expected. Even in this case,the solidus line temperatures of the Zn—Al-base solder is ensured to benot less than 280° C., so that there is no problem regarding thestrength required at high temperatures.

[0082] By plating the Zn—Al-base solder balls with Sn so that Sn phasewhich remains without being dissolved in the balls may be present, theSn phase acts to absorb the deformation, so that the rigidity of theZn—Al solder balls can be relieved. In order to further relieving therigidity, the Zn—Al-base solder balls may be used while mixing thereinplastic balls having a size of about 1 μm which are coated bymetallizing and soldering, so that the impact resistance thereof isimproved and the Young's modulus thereof decreases. Alternatively, byusing a paste in which the balls of Sn, In, and other materials, and therubber of the Sn-plated plastic balls are dispersed in the Zn—Al-base(for example, Zn—Al—Mg, Zn—Al—Ge, Zn—Al—Mg—Ge and Zn—Al—Mg—Ga) solderballs, it is possible to similarly improve temperature cycle resistanceand impact resistance, whereby the high reliability thereof can beensured. When only the Zn—Al-base solders are used, the balls are hard(about Hv 120-160) and the rigidity is large, so that there occurs sucha fear as a Si chip of a large size is broken. For removing this fear,the layers of soft, low-meling point Sn and In are made to be presentaround the balls, and the rubber is dispersed among the balls, wherebythe deformability is ensured and the rigidity decreases.

[0083] Embodiment 10

[0084]FIG. 5(a) to FIG. 5(c) show an example in which a relatively smalloutput module used for signal-processing in portable cellular phones,which module has such a large square shape as one side thereof is largerthan 15 mm in length, are mounted to a printed circuit board by a flatpack type package structure in which a difference in thermal expansioncoefficient between the module and the substrate is relieved by leads.In this type of structure, it is usual to adopt the system that the rearface of each of circuit elements is die-bonded to a junction substrateexcellent in thermal conductivity and they are connected to the terminalof the junction substrate by wire bonding. Regarding this system, thereare many examples in which a MCM (multi chip module) design is adoptedin which there are located several chips and chip parts such as ofresistors and capacitors arranged around each of the chips. Aconventional HIC (hybrid IC) or power MOSIC are the representativeexamples thereof. As an available module substrate, there are Sithin-film substrate, an AlN substrate having a low thermal expansioncoefficient and a high thermal conductivity, a glass ceramic substrateof a low thermal expansion coefficient, an Al₂O₃ substrate whosecoefficient of thermal expansion is close to that of GaAs, and ametal-core organic substrate of Cu or other material which has high heatresistance and improved thermal conduction.

[0085]FIG. 5(a) shows an example in which Si chips 8 are mounted on anSi substrate 35. Since resistors, capacitors, and other components canbe formed in thin films on the Si substrate 35, higher-density mountingis possible. In this example, a flip chip mounting structure of the Sichips 8 is shown. It is also possible to adopt such a system as Si chipsare bonded by die bonding while the terminals are connected by wirebonding. FIG. 5(b) shows another example in which the mounting on theprinted circuit board 49 is of a QFP-LSI type module structure and softCu-base leads 29 are adopted. It is general to perform the metallizingon the Cu leads 29 by Ni/Pd, Ni/Pd/Au, Ni/Sn, or other materials. Thebonding of the leads 29 and an Si substrate 35 is performed by heatingunder pressure by use of the paste relating to the invention. As regardsthe leads, it is possible to adopt a method in which the leads aresupplied as a row of a straight line by means of a dispenser or in whichthe supply of the material thereof is performed by printing regardingeach of the terminals and the leads are formed by performing theseparating thereof regarding individual terminals through the heatingunder pressure. The Au or Cu bumps 18 of each of the Si chips are bondedby supplying the paste relating to the invention to the junctionsubstrate 35. Alternatively, it is possible to perform the Au—Sn bondingor Cu—Sn bonding by plating with Sn the terminals located on thesubstrate side. Furthermore, as still another bonding method, in a casewhere Au ball bumps are used while Sn-plated terminals are provided onthe substrate, the Au—Sn bonding is obtained by a thermocompressionbonding technique, so that resultant junctions come to adequatelywithstand a reflow temperature of 250° C. Further, it is also possibleto use a heat-resistant, electrically conductive paste. For theprotection of the chips, there is provided on each of the chips asilicone gel 26, or an epoxy resin containing a filler and/or a rubbersuch as a silicone which epoxy resin has a low thermal expansioncoefficient and flexibility of a certain level while maintaining aflowability and a mechanical strength after setting, or a siliconeresin, thereby making it possible to protect and reinforce the chipsincluding the terminal portions of the leads. This enables lead-freebonding by the temperature hierarchy, the realization of which has beendesired.

[0086] In a case where there is used a thick film substrate such as anAlN substrate, a glass ceramic substrate or an Al₂O₃ substrate insteadof using the Si substrate, the mounting of the resistors, capacitors, orother components, as the parts of the chips becomes basic. Further,there is available a forming method in which laser trimming is performedwhile using a thick-film paste. In the case of resistors and capacitorsformed by the thick-film paste, it is possible to adopt the samemounting system as in the above Si substrate.

[0087]FIG. 5(b) shows another system comprising the steps of mountingthe chips 8 of Si or GaAs, with its face up, on an Al₂O₃ substrate 19excellent in thermal conductivity and in mechanical properties,performing the bonding thereof under pressure by means of a pulseresistance heating body, performing the reflow bonding of chip parts,performing the cleaning thereof, and performing the wire bonding. Inthis case, resin encapsulation is a general practice similarly to thecase of FIG. 5(a). The resin used therein is, similarly to the case ofFIG. 5(a), an epoxy resin of low thermal expansion coefficient in whicha quartz filler and rubber such as a silicone rubber are dispersed andwhich can reduce thermal impacts, or a silicone resin, or a resin inwhich the two of the resins are mixed in some states or forms. In thissystem, a large substrate of an undivided state is used insofar as themounting of the chips and the chip parts are concerned, and the largesubstrate is divided thereafter, and each of the divided portions iscovered with a resin after the lead bonding. The coefficients of thermalexpansion of GaAs and Al₂O₃ are close to each other, the paste solder ofthe invention containing about 50% Cu, and besides the bonding ispreformed through the structure of the bonded Cu particles, so thatthere is obtained a structure having excellent thermal conductivity. Tofurther improve the heat dissipation, thermal vias are provided underthe metallized layer formed immediately under the chip, thereby makingit possible to also dissipate heat from the back side of the substrate.The supply of the paste relating to the invention to these terminals isperformed by printing or by means of the dispenser. The paste relatingto the invention can be also used in solder junctions 33 that providesbonding between the lead 29 and the Al₂O₃ substrate 19.

[0088] In the case of the bonding of Al fins, if a non-cleaning type ispossible, there is available a system comprising the steps of supplyingthe paste in a shape surrounding the fins by means of a dispenser orprinting, and performing the bonding under pressure by means of theresistance heating body or a laser or a light beam, or other means, orbonding by one operation simultaneously with the chip parts by thereflow. In the case of Al materials, plating with Ni, or othermaterials, is performed as metallizing. In the case of the fin bonding,in order to realize the non-cleaning type, Al is worked to be in a foilshape and the foil thus obtained is bonded under pressure in an N₂atmosphere by means of the resistance heating body.

[0089]FIG. 5(c) shows a part of a module structure in which electronicparts are mounted on a metal-core substrate having a metal 39 thereinand are encapsulated with an Al fin 31. A chip 13 may have a face-downstructure and may be directly bonded to the metal 39 of the metal coreby installing dummy terminals 45 for heat dissipation. The bonding isperformed by LGA (lead grid array) system, the pads of a chip-side beingmade of Ni/Au or Ag—Pt/Ni/Au, the pads of a substrate-side being made ofCu/Ni/Au, and these are bonded to each other by use of the pasterelating to the invention. In a case of using a polyimide substrate thathas a low thermal expansion and a heat resisting property or using abuilt-up substrate having similarly thereto a heat resisting property,the module mounting with the temperature hierarchy can be performed inwhich the semiconductor devices 13 are directly mounted by use of apaste 36 relating to the invention. In the case of a chip of high heatgeneration, it is also possible that the heat is conducted to the metal39 through the thermal vias. Since in each of the thermal vias Cuparticles contact with each other are present, the heat isinstantaneously conducted to the metal, that is, this structure isexcellent in thermal conductivity. In this case, also regarding theportions where the cap 31 is bonded, the bonding is performed by use ofthe paste 31 relating to the invention. The paste portions 36 can beprinted in one operation.

[0090] As an example of applying the embodiment to a circuit element,the RF module was described above, however, the invention can also beapplied to any one of an SAW (surface acoustic wave) device structureused as a band pass filter for various types of mobile communicationequipments, a PA (high-frequency power amplifier) module, a module formoitoring a lithium cell, and other modules and circuit elements. Asregards the product field in which the solder of the invention can beapplied is not limited to portable cellular phones including mobileproducts, nor to notebook personal computers, and can be applied tomodule-mounting parts capable of being used in new household appliances,in the digitization age. Needless to say, the solder relating to theinvention can be used for the temperature-hierarchical bonding by use ofa Pb-free solder.

[0091] Embodiment 11

[0092]FIG. 6 shows an example of the application of the invention to ausual plastic package. Conventionally, the rear face of an Si chip 25 isbonded to a tab 53 of 42 Alloy by use of an electrically-conductivepaste 54. The circuit element is connected to each of leads 29 by wirebonding while using a gold wire 8, and is molded with a resin 5. Afterthat, the leads are plated with Sn-based solder corresponded to thePb-free bonding design. Conventionally, a eutectic Sn-37Pb solder with amelting point of 183° C. was able to be used for the mounting on aprinted circuit board and, therefore, it was possible to perform thereflow bonding at 220° C. maximum. However, in the case of the Pb-freebonding, since the reflow bonding is performed by use of theSn-3Ag-0.5Cu solder (melting point: 217-221° C.), the reflow temperaturebecomes about 240° C., that is, the maximum temperature becomes higherby about 20° C. than that of the conventional technique. Thus, insofaras a conventionally used heat-resistant, electrically-conductive pasteused for the bonding between the Si chip 25 and the tab 53 of 42-Alloyis concerned, the bonding strength at the high temperature decreases,and there occurs such a fear as the reliability thereof is affect.Therefore, by using the paste relating to the invention in place of theelectrically-conductive paste, it becomes possible to perform thePb-free bonding at about 290° C. with respect to the die bonding. Thisapplication to a plastic package can be applied in all plastic packagestructures in which an Si chip and a tab are bonded together. As for theshape of the leads, structurally there are the gull wing type, the flattype, the J-lead type, the butt-lead type and the leadless type. It isneedless to say that the invention can be applied to all of the types.

[0093] Embodiment 12

[0094]FIG. 7(a) to FIG. 7(c) show more specific examples in which theinvention is applied to the mounting of RF modules for high frequencies.FIG. 7(a) is a sectional view of the module and FIG. 7(b) is a plan viewof the module in which an Al fin 31 on the top face is removed.

[0095] In an actual structure, several MOSFET elements each comprising achip 13 with a size of 1×1.5 mm which generates radio waves are mountedwith face-up bonding in order to adapt to multi-band design, and besidesthere is formed, by parts 17 such as resistors and capacitors, aroundthe MOSFET parts a high-frequency circuit for efficiently generating theradio waves. Chip parts are also miniaturized and 1005, 0603, are used.The module is about 7 mm long and about 14 mm wide and is miniaturizedwith high-density mounting.

[0096] In this embodiment, only the functional aspect of the solder istaken into consideration, and there is described a model in which onecircuit element and one chip are mounted as the representatives thereof.In this case, as described hereinbelow, the chip 13 and circuit element17 are bonded to a substrate 43 by the solder paste relating to theinvention. The terminals of the Si (or GaAs) chip 13 are bonded to thepads of the substrate 43 by wire bonding 8, and in addition areelectrically connected, via through holes 44 and interconnector 45, toterminals 46 that provide the external connection on the rear face ofthe substrate. The chip part 17 is solder-bonded to the pads of thesubstrate and is further electrically connected, via the through holes44 and the interconnector 45, to the terminals 46 that provide theexternal connection on the rear face of the substrate. The chip 13 isoften coated with a silicone gel (omitted in this figure). Under thechip 13 are provided thermal vias 44 for heat dissipation, which areguided to a terminal 42 for heat dissipation on the rear face. In thecase of a ceramic substrate, the thermal vias are filled with athick-film paste of a Cu-base material excellent in thermalconductivity. When an organic substrate that is relatively inferior inheat resistance is used, by using the paste relating to the invention itis possible to perform the soldering in the range of 250° C. to 290° C.for the bonding of the rear face of the chip, the bonding of the chipparts, and in the thermal vias. Furthermore, the Al fin 31 covering thewhole module and the substrate 43 are fixed together by caulking, orother means. This module is mounted by solder-bonding the terminals 46,that provide an external connection, to a printed circuit board, and inthis case the temperature-hierarchical bonding is required.

[0097]FIG. 7(c) shows an example in which, besides this FR module, asemiconductor device of BGA type and a chip part 17 are mounted on aprinted circuit board 49. In the semiconductor device, a semiconductorchip 25 is bonded, in a face-up state, to a junction substrate 14 by useof the solder paste relating to the invention, the terminals of thesemiconductor chip 25 and the terminals of the junction substrate 14being bonded together by wire bonding, and the areas around the bondingportions are resin-encapsulated. For example, the semiconductor chip 25is die-bonded to the junction substrate 14 through the resistanceheating body by melting the solder paste at 290° C. for 5 seconds.Further, on the rear face of the junction substrate 14 is formed solderball terminals 30. For example, a Sn3Ag-0.5Cu solder is used in thesolder ball terminals 30. Moreover, also to the rear face of thesubstrate 49 is solder-bonded a semiconductor device (in this example,TSOP-LSI), which is an example of so-called double-sided mounting.

[0098] As a method of the double-sided mounting, for example, aSn-3Ag-0.5Cu solder paste is first printed in pad portions 18 on thesubstrate 49. Then, to perform solder bonding from the side of themounting face of a semiconductor device such as TSOP-LSI50, TSOP-LSI50is located and the reflow bonding thereof is performed at 240° C.maximum. Next, chip parts, a module and a semiconductor are located andthe reflow bonding thereof is performed at 240° C. maximum, wherebydouble-sided mounting is realized. It is usual to first perform thereflow bonding regarding light parts having heat resistance and then toperform the bond of heavy parts that have no heat resistance, as in thiscase of the above example. In performing the reflow bonding at a laterstage, it is necessary that the solder of the first bonded parts be notallowed to fall, and it is ideal to prevent the solder from beingre-melted.

[0099] In the case of the reflow and the double-sided mounting by thereflow, there occurs such a case as the temperature of the jointsalready mounted on the rear face exceeds the melting point of thesolder. However, in most cases, there is no problem in case the mountedparts do not fall. In the case of the reflow, the temperature differencebetween the upper and lower faces of the substrate is small, so that thewarp of the substrate is small and light parts do not fall because ofthe action of the surface tension even if the solder is melted. Althoughthe combination of the Cu balls and Sn was described above in therepresentative examples relating to the invention, it is needless to saythat the invention similarly applies to other combinations recited inthe claims.

[0100] Embodiment 13

[0101] Next, to further reduce the cost of a RF module, a resinencapsulation method by the paste relating to the invention is describedbelow.

[0102]FIG. 8(a) shows the RF module assembling steps of the resinencapsulation method and FIG. 8(b) shows the secondary mounting andassembling steps for mounting a module on a printed circuit board. FIG.9(a) to FIG. 9(d) are sectional model drawings in which the sequence ofassembling in the RF module assembling steps of FIG. 8(a) is shown. Thesize of an Al₂O₃ multilayer ceramic substrate 43 of a square shape is aslarge as 100 to 150 mm in one side, and the Al₂O₃ multilayer ceramicsubstrate 43 is provided with slits 62 for break so that it can bedivided to each of module substrates. Cavities 61 are formed in theposition where each of Si chips 13 on the Al₂O₃ multilayer ceramicsubstrate 43 is to be die-bonded, and each of the surfaces of thecavities 61 is plated with a thick-Cu-film/Ni/Au or Ag—Pi/Ni/Au. Justunder the die-bond are formed a plurality of thermal vias (filled withCu thick-film conductors) 44, which are connected to pads 45 formed onthe back side of the substrate to thereby dissipate heat through amultilayer printed circuit board 49 (FIG. 9(d)). This enables the heatoccurring from a high-output chip of several watts to be smoothlydissipated. An Ag—Pt thick-film conductor was used to form the pads ofthe Al₂O₃ multilayer substrate 43. Alternatively, a Cu thick-filmconductor may be used in dependence on the type and fabrication methodof a junction substrate (made of Al₂O₃ in this example), or it ispossible to use a W—Ni conductor or Ag—Pd conductor. The pad portions ineach of which a chip part is mounted are made of the plating ofthick-Ag—Pt-film/Ni/Au. As regards the pads formed in the rear face ofthe Si chip, the thin film of the Ti/Ni/Au is used in this example,however, the pads are not limited to this structure, and such a thinfilm of Cr/Ni/Au, as to be usually used can be also used.

[0103] After the die bonding of the Si chip 13 and the reflow of a chippart 17 (which will be described later in detail), wire bonding 8 isperformed after the cleaning of the Al₂O₃ multilayer substrate (FIG.9(b)). Further, a resin is supplied thereto by printing and a sectionshown in FIG. 9(c) is obtained. The resin, which is a silicone resin orlow-elasticity epoxy resin, is printed by means of a squeegee 64, asshown in FIG. 10, so as to cover the Al₂O₃ multilayer substrate 43 withthe resin by one operation, whereby a single-operation encapusulatedportion 73 is formed on the Al₂O₃ multilayer substrate 43. After thesetting of the resin, identification marks are put by a laser, and acharacteristics check is conducted after the dividing of the substrate.FIG. 11 is a perspective view of a module which was completed by thesteps of dividing the Al₂O₃ multilayer substrate, mounting it on aprinted circuit board and performing the reflow thereof. The module ismade to have a LGA structure, so that it becomes possible to perform ahigh-density mounting on a printed circuit board.

[0104] Next, the above description is supplemented by referring to thesequence of steps of the RF module assembling shown in FIG. 8(a). Thepaste relating to the invention is supplied to the chip part byprinting, and this paste is supplied by means of a dispenser withrespect to the chips to be mounted on the cavities. First, passivedevices 17 such as chip resistors and chip capacitors, are mounted.Next, the 1×1.5-mm chip 13 is mounted and, at the same time, the diebonding thereof is performed by lightly and uniformly pressing the Sichip by means of a heating body at 290° C. to thereby perform theplanarization thereof. The die bonding of the Si chip and the reflow ofthe chip parts are performed in a series of steps mainly by the heatingbody located under the Al₂O₃ multilayer substrate. To eliminate voids,Sn-plated Cu balls were used. At 290° C., the Cu balls soften a littleand Sn improves fluidity at the high temperatures, thereby activatingthe reaction between Cu and Ni. In this case, the compound is formed incontact portions where Cu particles are in contact with each other andwhere Cu particles and metallized portions are in contact with eachother. Once the compounds are formed, they do not re-melt even at thereflow temperature of 250° C. because of their high melting points.Further, because the die bonding temperature is higher than thesecondary reflow temperature, Sn wets and spreads out sufficiently tothereby becomes the compound. As the result thereof, during thesecondary reflow, the compound layers come to provide a sufficientstrength at the high temperatures, so that the Si does not move even inthe resin-encapsulated structure. Further, even in a case where thelow-melting point Sn remelts, it does not flow out because it hasalready been subjected to the heat history of the higher temperatures.For these reasons, the Si chip remains stationary during the secondaryreflow, so that the module characteristics are not affected by there-melting of Sn.

[0105] Next, there are below described influences caused by the resinwhile comparing the case of the paste relating to the invention withthat of conventional the Pb-base solder (which makes it possible toperform the reflow at 290° C.

[0106] In FIG. 12(a) and FIG. 12(b) there is shown a model of aphenomenon of a short circuit caused in a chip part 17 by theflowing-out 71 of a conventional Pb base solder (having a solidus linetemperature of 245° C.) in a case where the secondary reflow (220° C.)for peforming the bonding to a printed circuit board is performed (whichis similar to the mounting state of FIG. 11 and the composition of thesolder 30 is a Sn—Pb eutectic). In a case of the module encapsulated bya filler-containing, high-elasticity epoxy resin 68 (that is, in thecase of a chip part plated with Sn or Sn—Pb, which is usually used formetallizing, the melting point at which this solder remelts decreases toabout 180° C. because of the formation of a eutectic phase of the Sn—Pb,the circuit short caused under the pressure of this resin by using ofthe modulus of elasticity of the resin at 180° C. at which the solderflows out is 1000 MPa. Although the melting point of the Pb-base solderis originally the solidus line temperature of 245° C., it decreases toabout 180° C. because the pads of the chip part are plated with theSn—Pb solder and because the substrate side is plated with Au.Therefore, the Pb-base solder is in a remolten state during thesecondary reflow (220° C.). When the Pb-base solder changes from thesold to the liquid, a volume expansion of 3.6% occurs abruptly in thesolder. Both of the remelting expansion pressure 70 of the Pb-basesolder that forms a fillet on the side of the chip part and the resinpressure 69 balance with each other with large stress and exfoliate theinterface formed between the top surface of the chip and the resin,which is a structurally weak portion, causing the solder to flow out. Asthe result, the short circuits to the pads on the opposite side occurredat a high probability (70%). It is also found that the incidence of thisshort-circuit phenomenon can be lowered by the lowering of the modulusof elasticity of a resin defined at a high temperature (180° C.). Sincethere is a limit as regards the softening of epoxy resins, the researchwas made while raising the modulus of elasticity by adding a filler to asoft silicone resin. As the result, it is found that the outflow of thesolder will not occur when the elastic modulus at 180° C. is not morethan 10 MPa. When the modulus of elasticity was increased to 200 MPa at180° C., short circuits occurred at the probability of 2%. In view ofthe foregoing, it is necessary that, in a solder structure whichremelts, the modulus of elasticity of the resin be not more than 200 MPaat 180° C.

[0107] Then, the influence caused by the outflow regarding the pastestructure of the present invention is shown in FIG. 13 while comparingit with a conventional solder. As described above, when bonding isperformed by use of the paste relating to the invention, the volumeoccupied by the Sn in the molten portion is about a half and, partlybecause the expansion value of Sn itself is small, the volume expansionratio of the solder becomes 1.4%, which is 1/2.6 as low as that of thePb-base solder. Further, as illustrated by the model shown in FIG. 13,the Cu particles are bonded together in a point-contact state, thepressure of the resin is balanced by the reaction of the Cu particleseven at the time of the melting of Sn, so that no crushing of thesoldered portion occurs, that is, a phenomenon quite different from thecase of the molten solder is expected. In other words, it is expectedthat the probability of the occurrence of the short circuits betweenelectrodes due to the outflow of Sn is low. Thus, the outflow of soldercan be prevented even with an epoxy resin which is so designed that itbecomes somewhat soft even when a filler is added. From the result ofFIG. 13, in assuming that the complete melting of Sn occurs and that amodulus of elasticity of the resin in inverse proportion to the volumeexpansion ratio is allowed, the allowable modulus of elasticity of theresin becomes 500 MPa. Actually, the effect of the reaction of Cuparticles can be expected, so that it is expected that no outflow occurseven with a resin having a high modulus of elasticity. In a case wherethe use of an epoxy resin is possible, the dividing of a substrate canbe mechanically performed, and it becomes unnecessary to make cuts inthe resin by means of a laser, or other system, so that the productivityand efficiency are also improved.

[0108] The above module mounting can also be applied to other ceramicsubstrates, organic metal-core substrates and built-up substrates.Furthermore, the chip element can be bonded both in a face-up manner andin a face-down manner. As regards the module, the invention can also beapplied to surface-acoustic-wave (SAW) modules, power MOSIC modules,memory modules, multichip modules, etc.

[0109] Embodiment 14

[0110] Next, there is described an example of application of theinvention to the resin package of a high-output chip such as amotor-driver IC. FIG. 14(a) is a plan view of the high-output resinpackage in which a lead frame 51 and a thermal-diffusion plate 52 arebonded together and caulked. FIG. 14(b) is a sectional view of thepackage. FIG. 14(c) is a partially enlarged view of a circle portion inFIG. 14(b). In this example, a semiconductor chip 25 is bonded to athermal-diffusion plate (heat sink) 52 by use of the solder pasterelating to the invention. The lead 51 and the terminals of thesemiconductor chip 25 are bonded together by wire bonding and are resinencapsulated. The lead is made of a Cu-base material.

[0111]FIG. 15 is a flow chart of the steps of the high-output resinpackage. First, onto the lead frame 51 and the thermal-diffusion plate52 both joined by caulking is die-bonded a semiconductor chip 25 bysupplying a solder paste 3. The semiconductor chip 25 bonded by the diebonding is further wire bonded, as shown in the figure, by means of thelead 51, a gold wire 8. After that, the resin encapsulation is performedand the Sn-base solder plating is performed after the dam cutting. Then,lead-cutting and lead-forming are performed, the cutting of thethermal-diffusion plate being performed, whereby the package iscompleted. The back-side pads of the chip can be metallized by amaterial usually used, such as Cr—Ni—Au, Cr—Cu—Au and Ti—Pt—Au. Even ina case where the Au content is large, good results are obtained insofaras a Au-rich compound having a high Au—Sn melting point is formed. Asregards the die bonding, it is performed by means of a resistanceheating body with an initial pressure of 1 kgf, at 300° C. for 5 secondsafter the supply of the solder by printing.

[0112] For a large chip, it is preferred that, in the case of anespecially hard Zn—Al-base solder, high reliability be ensured by addingrubber and a low-expansion filler.

[0113] Embodiment 15

[0114]FIG. 16(a) to FIG. 16(d) show, regarding examples of BGA and CSP,a package of a chip 25 and a junction substrate 14 which package isobtained by the temperature-hierarchical bonding of the Pb-free solderby use of Cu balls 80 capable of keeping a strength even at 270° C.Conventionally, the temperature-hierarchical bonding was performed byuse of high melting Pb-(5-10)Sn solders for bonding a chip and a ceramicjunction substrate together. However, when Pb-free solders are to beused, there is no means that replace the conventional one. Therefore,there is provided such a structure as, by use of a Sn base solder and acompound occurring thereby, a bonded portion is not melted at the timeof the reflow to thereby maintain a bonding strength even when theportion of the solder is melted. FIG. 16(a) shows a sectional model ofBGA/CSP, in which an organic substrate such as a built-up substrate wasused as the junction substrate although a built-up substrate, metal-coresubstrate, ceramic substrate etc., can be considered. As regards theshape of a bump, there are a ball bump (FIG. 16(b)), a wire bond bump(FIG. 16(c)) and a Cu-plated bump of a readily deformable structure(FIG. 16(d)). The external connection terminals are Cu pads orSn—Ag—Cu-base solder portions 30 fed through balls or paste onNi/Au-plating portions 83.

[0115] In the case shown in FIG. 16(a), it becomes possible to obtainbonding capable of withstanding the reflow by the steps of: feeding Snonto a thin-film pads 82 on the side of the Si chip 25 by means of vapordeposition, plating, a paste, or the composite paste including metalballs and solder balls; thermally pressure-bonding thereto metal balls80 such as balls of Cu, Ag, Au, or Au-plated Al balls, or metallizedorganic resin balls to thereby form an intermetallic compound 84 with Snat contact portions 84 in contact with the thin-film pad material (Cu,Ni, Ag) or in the vicinity of this contact portion. Next, the ball padsformed on the above chip is positioned on the pads of a junctionsubstrate (an Al₂O₃, AlN, an organic, a built-up substrate or ametal-core substrate), to which pads a paste comprising metal balls, asolder (Sn, Sn—Ag, Sn—Ag—Cu, Sn—Cu, or those containing at least one ofIn, Bi and Zn) and balls is supplied beforehand, and is thermallypressure-bonded, whereby similarly a compound 84 of the pads 83 of thejunction-substrate and Sn is formed to thereby make it possible toprovide a structure capable of withstanding 280° C. Even when the bumpheight varies, the variation is compensated for by the composite paste.Thus, it becomes possible to obtain BGA or CSP or high reliability inwhich stress burden to each of the solder bumps and to the Si chip padsis small with the result that the service life of the bumps is increasedand in which, for the protection against mechanical impact, the fillingis formed with a solvent-free resin 81 superior in fluidity havingYoung's modulus in the range of 50 to 15000 Mpa and a coefficient ofthermal expansion of 10 to 60×10−6/° C.

[0116] The processes of FIG. 16(b) to FIG. 16(d) are described below.

[0117]FIG. 17(a) to FIG. 17(c) show a bonding process for bonding the Sichip 25 and the junction substrate 14 together, by the system of the Cuball 80 shown in FIG. 16(b). Although the electrode terminals 82 on thechip 20 are made of Ti/Pt/Au in this case, the material is not limitedto the Ti/Pt/Au. In the stage of wafer process, an Sn plating, anSn—Ag—Cu-base solder or a composite paste 85 containing metal balls andsolder balls is fed to the thin-film pads 82 formed on each chip. Au isprovided mainly for the prevention of surface oxidation and is as thinas not more than 0.1 μm. Therefore, Au dissolves in the solder in asolid solution state after melting. As regards the Pt—Sn compoundlayers, there are present various compounds such as Pt₃Sn and PtSn₂.When the diameter of the ball 80 is large, it is desirable to adopt aprinting method capable of supplying a thick solder 85 for fixing theballs. Alternatively, balls which are solder plated beforehand may beused.

[0118]FIG. 17(a) shows a state in which a 150-μm metal ball (Cu ball) 80is positioned and fixed by a metal-mask guide after the application of aflux 4 onto the terminal plated with Sn 23. To ensure that all balls onthe wafer or chip come into positive contact with the central part ofthe thin-film pads 82, melting under pressure is performed at 290° C.for 5 seconds by means of a flat pulse-current resistance heating body,or other means. Due to size variations of Cu balls in the chip, someballs do not come into contact with the pad portions, however, in a casewhere these balls are close to the pad portions, the possibility of theforming of an alloy layer becomes high, although this depends on theplastic deformation of Cu at high temperatures. Even if there are a fewbumps which are in contact with the pad portions via an Sn layer withoutthe forming of the alloy layer, there is no problem insofar as themajority of the bumps form the alloy layer. In the case of the compositepaste 34, even when the Cu ball 80 does not come into contact with thepad portion, the pad portions are connected to the Cu ball by the alloylayer formed after the bonding and the strength is ensured even at hightemperatures.

[0119] The section of the electrode portion after melting is shown inFIG. 17(b). The Cu ball comes into contact with the terminal, and acontact portion 84 is bonded by compounds of Pt—Sn and Cu—Sn. Even in acase where the contact portions are not bonded completely by thecompounds, the alloy layer grows because of heating or pressurization,in succeeding steps with the resut that the joining thereof is achieved.Although Sn fillets are formed in the surrounding area, Sn often doesnot always wet to spread on the whole Cu. After the bonding of the ball,cleaning is performed for each wafer or chip (in the case of a wafer,the wafer is cut for each chip), the back side of the chip being thenattracted by means of the pulse-current resistance heating body, theball terminal being positioned and fixed to a composite past 36 formedon the electrode terminal 83 of a built-up junction substrate 14, andmelting under pressure is performed at 290° C. for 5 seconds whilespraying a nitrogen gas. A flux may be used when no resin-filling isperformed in the succeeding step.

[0120]FIG. 17(c) shows a section after the melting under pressure. Fromthe electrode terminal 82 on the chip side to the electrode terminal 83on the junction substrate side, all of the high-melting metals and theintermetallic compounds 41 are connected to each other in succession, sothat no exfoliation occurs even in the succeeding reflow step. Due toheight variations of the ball bumps, some bumps do not come into contactwith the pads on the junction substrate. However, because these ballbumps are connected by the intermetallic compounds 84, there occurs noproblem even during the reflow.

[0121]FIG. 16(c) shows an example in which a wire bonding terminal(Cr/Ni/Au) 48 on the Si chip side and a wire bumping terminal 86 of Cu,Ag or Au, or other materials, are bonded together by thermal pressurebonding (in some cases, an ultrasonic wave may be applied thereto). Thefeature of the wire bumping terminal are its shape deformed bycapillaries and its torn neck portion. Although height variations in thetorn neck portion are large, in some of them the irregular heights aremade to be flat during the 1pressurization and, since it is bonded bythe mixture paste, there occurs no problem. As the material for the wirebumping terminal, there are materials of Au, Ag, Cu and Al which wetwell with Sn and are soft. In the case of Al, the use is limited tosolders which wet with Sn and the range of the selection is narrow,however, it is possible to use Al. Similarly to the case shown in FIG.16(b), since the cleaning of a narrow gap causes diifficulties inoperation, it is made to be a premise that a non-cleaning process isused. After the positioning, it becomes possible to similarly form theintermetallic compound 84 of both of Sn and the pad of the junctionsubstrate by performing thermal pressure bonding while spraying anitrogen gas, an intermetallic compound 41 of the junction substrateelectrode with Sn can be formed similarly, so that a bonding structurecapable of withstanding 280° C. can be obtained similar to the case ofFIG. 16(b).

[0122] The process for producing the structure of FIG. 16(d) is shown inFIGS. 18(a) and 18(b). The process is a system in which, in the waferprocess, the relocation is performed by a Cu terminal 87 and a polyimideinsulating film 90 on a semiconductor device of Si chip 25 and in whichbumps are then formed by Cu plating 88. By use of a photoresist 89 andCu-plating technology there is provided a Cu-plated bump structure 91which is not a simple bump but has a thin neck portion readilydeformable under stresses in a plane direction. FIG. 18(a) is asectional drawing of a model formed in the wafer process, in which, inorder to ensure that no stress concentration occurs on the relocatedterminal, a readily deformable structure is formed by use of thephotoresist 89 and plating and thereafter the photoresist is removed sothat a Cu bump may be formed. FIG. 18(b) shows the section of a bondingportion formed between the Cu bump 91 and the Cu terminal through theintermetallic compound 84 of Cu₆Sn₅ by the steps of coating the junctionsubstrate 14 with a composite paste of Cu and Sn, positioning the Cubump 91 of the chip, and pressurizing and heating it (at 290° C. for 5seconds) in a nitrogen atmosphere without using a flux.

[0123] Embodiment 16

[0124] Next, to examine an appropriate range of the ratio of the metalballs included in the solder paste (Cu was selected as a representativecomponent) to solder balls (Sn was selected as a representativecomponent), the weight ratio of Sn to Cu (Sn/Cu) was varied. The resultof the examination is shown in FIG. 19. As regards a method ofevaluation, the section of a bonding portion after the reflow wasobserved and appropriate amounts of the mixed components were examinedfrom the states of the contact and/or the approaching of Cu particles.The flux used here was a usual non-cleaning type. As regrads theparticle sizes of Cu and Sn, relatively large particles of 20 to 40 μmwere used. As the result, it is found that the Sn/Cu ratio range ispreferably in the range of 0.6 to 1.4 and more preferably 0.8 to 1.0.Unless the particle size is 50 μm or less at most, it is impossible toadapt to the fine design (regarding the pitch, the diameter of each ofthe teminals, and the space therebetween), and the level of 20 to 30 μmis readily used. The fine particles of 5 to 10 μm is also used asparticle size that provide a margin with respect to the above finedesign. However, in the case of an excessively fine size, since thesurface area increases and since the reducing capability of the flux islimited, there occur such problems as the solder balls remain and as thecharacteristic of the softness of Sn is lost due to the acceleration ofthe Cu—Sn alloying. The solder (Sn) does not relate to particle sizebecause it eventually melts, however, it is necessary that in a pastestate, Cu and Sn be uniformly dispersed, so that it is basic to make theparticle size of the two be in the same level. Further, it is necessaryto plate the surfaces of the Cu particles with Sn to a coating thicknessof about 1 μm so that the solder becomes wettable. This enables theburden on a flux to be reduced.

[0125] In order to reduce the rigidity of the composite solder, it iseffective to disperse among the metal and solder balls the soft,metallized plastic balls. In particular, in the case of a hard metal,this is effective in improving the reliability because the soft plasticballs act to reduce the deformation and thermal impact. Similarly, bydispersing substances of low thermal expansion, such as invar, silica,AlN and SIC, which are metallized in the composite solder, stresses inthe joint can be reduced, so that the high reliability can be expected.Incidentally, the alloy is noted as a new material that lowers themelting points, not for reasons of the mechanical properties, however,because the alloy is in general a hard material, it can be improved bydispersing the soft metal balls, such as metallized Al or the plasticballs.

[0126] The effects obtained by the representative essential features ofthe invention are briefly described below. According to the invention,it is possible to provide the solder capable of maintaining strength athigh temperatures in the temperature-hierarchical bonding. Further,according to the invention, it is possible to provide a method oftemperature-hierarchical bonding in which the solder capable ofmaintaining strength at high temperatures is used. Moreover, accordingto the invention, it is possible to provide the electronic device whichis bonded by use of the solder capable of maintaining strength at hightemperatures.

What is claimed is:
 1. A solder comprising balls selected from the group consisting of Sn balls and In balls, and metal balls having a melting point higher than that of each of the Sn balls and the In balls.
 2. A solder according to claim 1 wherein the metal balls comprise Cu balls.
 3. A solder according to claim 1 wherein the metal balls comprise Al balls.
 4. A solder according to claim 1 wherein the metal balls comprise Ag balls.
 5. A solder according to claim 1 wherein the metal balls comprise Au balls.
 6. A solder according to claim 1 wherein the metal balls comprise balls of one kind selected from the group consisting of Cu balls, Cu—Sn alloy balls, Ni—Sn alloy balls, Zn—Al based alloy balls, and Au—Sn based alloy balls.
 7. A solder according to claim 1 further comprising plastic balls.
 8. A solder according to claim 1 further comprising metallized particles of at least one kind selected fomi the group consisting of invar, silica, alumina, AlN, and SiC.
 9. A solder according to claim 1 further comprising Bi.
 10. A solder according to claim 1, substantially all of said metal balls being provided on a surface thereof with a coating of a metal selected from the group consisting of Sn and a Sn alloy.
 11. A solder according to claim 1, substantially of said metal balls having a size ranging from 5 μm to 10 μm.
 12. A solder according to claim 1, substantially of said metal balls having a size ranging from 20 μm to 40 μm.
 13. A solder according to claim 1, a weight ratio of said Sn balls or In balls to said metal balls being in a range of about 0.6 to 1.4.
 14. A solder according to claim 1, a weight ratio of said Sn balls or In balls to said metal balls being in a range of about 0.8 to 1.0.
 15. A solder according to claim 1, said Sn balls or In balls having a melting point lower than that of a Sn—Ag—Cu-based solder, said metal balls having a melting point higher than that of said An—Ag—Cu-based solder.
 16. A solder comprising Cu balls and Sn balls, said solder coming to have, at a temperature not less than the melting point of Sn, compounds including Cu₆Sn₅ so that said Cu balls are bonded to each other by said compounds including Cu₆Sn₅.
 17. A solder comprising Cu balls and Sn balls, said Cu balls making, at a time when said Sn ball is melted, spaces defined among said Cu balls which spaces are filled with said molten Sn, said solder coming to contain, at this time, compounds including Cu₆Sn₅ present on at least a part of a surface of substantially all of said Cu balls so that said Cu balls are bonded to each other by said compounds including Cu₆Sn₅.
 18. A solder according to claim 16, substantially all of said Cu balls having a size ranging from 5 μm to 10 μm.
 19. A solder according to claim 16, substantially all of said Cu balls having a size ranging from 20 μm to 40 μm.
 20. A solder according to claim 16, a weight ratio of said Sn balls to said Cu balls being in a range of about 0.6 to 1.4. 